1. Field of the Invention
The present invention relates to systems for feeding solutions to patients. More particularly, the present invention relates to a system and method for monitoring fluid pressures to ensure that the solution is properly fed to the patient by the enteral feeding pump or similar device. Specifically, the invention relates to the use of optical pressure sensors to monitor fluid pressures and the presence of occlusions in the delivery set which may interfere with solution flow to the patient, and so that operation of the enteral feeding pump may be modified to compensate for the pressure and/or occlusion and thereby provide highly accurate solution delivery.
2. State of the Art
There are numerous situations in which a solution must be fed to a patient over a period of time. In some situations, the solution is provided directly into the blood stream of the patient. Saline solutions and medications supplied in such a manner are typically referred to as parenteral solutions. Because parenteral solutions are often necessary to hydrate the patient, or supply needed medications, occlusion of the infusion delivery set can be particularly dangerous to the patient. Thus, it is important to ensure that occlusions are not impeding solution flow. However, parenteral solutions typically have a very low viscosity in order to faciliate absorption into the blood stream. Therefore, the risk of occlusion of the infusion set is relatively small so long as the tube is not pinched shut by folding or some other external application of force.
In contrast to a parenteral system, an enteral feeding system is used to provide nutrient solutions to patients who, for one reason or another, are unable to eat for themselves. Such a system typically includes a pump which is attached to an input tube connected to a supply container and to an output tube which is connected to a patient. The pump draws nutrient solution from the supply container and delivers the solution to the patient. By adjusting the number of rotations of the motor, or the frequency of rotations, in the pump, an enteral feeding pump can adjust its output to deliver a predetermined amount of nutrient solution (or even medication) at a desired rate.
A significant problem with currently available enteral feeding systems, is that the intake and output tubes may become occluded. Unlike parenteral solutions, enteral feeding solutions have a relatively high viscosity, as they must carry sufficient nutrition to sustain the patient. Occlusion can occur, for example, if a fibrous substance is included in the enteral feeding solution and somehow combines to interfere with flow through the tube. Occlusion can also occur if a tube is bent sufficiently to interfere with flow therethrough, or if a roller clamp (as is commonly used for intravenous applications) is not sufficiently opened. Because of the viscosity of the solution, the amount of kinking of the tube or other interference required to interfere with solution flow is significantly less than that required in a parenteral infusion set.
If the intake tube becomes occluded, insufficient solution may be supplied to the pump, and thus to the patient. If the output tube becomes occluded, the flow of solution may be blocked, or the solution may be suddenly delivered at unusually high pressures. Additionally, medical personnel may fail to notice that the supply container is out of solution, or may not properly mount the intake and/or output tubes in the pump, thereby preventing the proper amount of solution from being delivered to the patient. Any of these scenarios can have tragic consequences if allowed to continue for a prolonged period of time.
Yet another concern with enteral feeding systems is that of viscosity of the solution and viscosity changes as a container full of solution is pumped to a patient. Knowing the viscosity of the fluid being pumped through the enteral feeding system is important because different viscosities are pumped at different rates by the enteral feeding pump. For example, a lower quantity of a highly viscous solution will be pumped by a given number of rotations of the enteral feeding pump motor than will be moved by the same pump when the solution has low viscosity. In other words, the amount of solution fed to the patient can differ substantially depending on the solution's viscosity. Thus, unless the pump is able to accurately determine and compensate for viscosity changes in the solution (i.e. by increasing or decreasing the rotations of the pump rotor in a given period of time), it is difficult to know exactly how much of the solution has been fed to the patient.
To overcome these concerns, there is a need for a system and method for determining flow discrepancies due to occlusions, viscosity (including changing viscosity) and/or improper fitting of pumps and intake/output tubes so that patients will not be endangered, and so that the proper amount of fluid will be delivered to the patient.
U.S. Pat. No. 5,720,721 (Feb. 24, 1998), which is expressly incorporated herein, provides a significant improvement in monitoring for enteral feeding pumps. The invention uses two pressure sensors to monitor viscosity and occlusions, and to enable the enteral feeding pump to compensate for the varying amount of solution which will pass through the pump with each rotation of the rotor. The pressure sensors engage the elastic tube of the infusion set and monitor changes in the strain on the infusion set by occlusions and viscosity changes. The strain information can then be processed by the pump and adjustments made to the number of rotations of the pump rotor to compensate. In the event that the occlusion is too severe to compensate by modification of the rotor rotations, the pump is shut down and an alarm signal generated so that replacement tubing may be provided.
While the pressure sensor system of U.S. Pat. No. 5,720,721 is a significant improvement over the art, it does have limitations. The pressure sensors described in the '721 patent are relatively expensive and must be properly mounted in the pump. Additionally, the person loading the pump must make sure that the upstream and downstream portions of the infusion set are properly loaded in the pump housing so that they engage the pressure sensors in the proper manner. Failure to properly load the infusion set can interfere with the functioning of the pressure sensors.
One manner for decreasing the costs of pressure sensors is to use optical sensors. While there are several methods for using optical sensors to determine the presence of occlusions, each has significant drawbacks. Some methods only allow the mechanism to determine when the pressure exceeds a certain threshold. This is done by detecting when the expanding tube of the infusion set engages a surface, thereby modifying reflection or refraction of light. Other methods require complex calculations of refraction indexes or otherwise provide relatively limited information on small pressure changes. Additionally, some methods can vary based on the material from which the infusion set is formed, or based on whether the tube of the infusion set is opaque or transparent.
In addition to the above, many mechanisms for monitoring pressure within an infusion set lack an inherent failure detector. For example, if a sensor is configured to sense only when the expanding infusion set tube engages a transparent surface, the failure to record a reflected signal may mean that the tube has not expanded. In certain situations, however, the lack of reflected signal could also mean that the sensor has failed and is either not sending the signal or is not receiving the reflected signal.
Thus, there is a need for an improved optical pressure monitoring system and method of use. Such an optical pressure monitoring system should be relatively inexpensive and easy to use. It should also provide highly accurate determination of pressure changes which indicate occlusions and/or viscosity changes. Furthermore, it should enable the use of infusion sets made from a variety of materials and without regard to whether the infusion set is formed of a tube which is transparent or opaque.